Computing devices, such as notebook computers, personal data assistants (PDAs), kiosks, and mobile handsets, have user interface devices, which are also known as human interface devices (HID). One user interface device that has become more common is a touch-sensor pad (also commonly referred to as a touchpad). A basic notebook computer touch-sensor pad emulates the function of a personal computer (PC) mouse. A touch-sensor pad is typically embedded into a PC notebook for built-in portability. A touch-sensor pad replicates mouse X/Y movement by using two defined axes which contain a collection of sensor elements that detect the position of a conductive object, such as a finger. Mouse right/left button clicks can be replicated by two mechanical buttons, located in the vicinity of the touchpad, or by tapping commands on the touch-sensor pad itself. The touch-sensor pad provides a user interface device for performing such functions as positioning a pointer, or selecting an item on a display. These touch-sensor pads may include multi-dimensional sensor arrays for detecting movement in multiple axes. The sensor array may include a one-dimensional sensor array, detecting movement in one axis. The sensor array may also be two dimensional, detecting movements in two axes.
One type of touchpad operates by way of capacitance sensing utilizing capacitance sensors. The capacitance, detected by a capacitance sensor, changes as a function of the proximity of a conductive object to the sensor. The conductive object can be, for example, a stylus or a user's finger. In a touch-sensor device, a change in capacitance detected by each sensor in the X and Y dimensions of the sensor array due to the proximity or movement of a conductive object can be measured by a variety of methods. Regardless of the method, usually an electrical signal representative of the capacitance detected by each capacitive sensor is processed by a processing device, which in turn produces electrical or optical signals representative of the position of the conductive object in relation to the touch-sensor pad in the X and Y dimensions. A touch-sensor strip, slider, or button operates on the same capacitance-sensing principle.
A first type of conventional touchpad is composed of a matrix of rows and columns. Within each row or column, there are multiple sensor elements. However, all sensor pads within each row or column are coupled together and operate as one long sensor element. A second type of conventional touchpad is composed of an XY array of independent sense elements, where each sensor element in a row or column is separately sensed. Here, each row and column is composed of multiple sensing elements, each capable of independent detection of a capacitive presence and magnitude. These may then be used to detect any number of substantially simultaneous touches.
The capacitive sensing systems used in interface devices such as touchpads generally operate by detecting changes in the capacitances of the capacitive sensors resulting from proximity or contact of an object with the sensor, however the ability to resolve changes in capacitance may be impaired if the changes in capacitance to be detected by the sensor are small relative to the capacitance of the sensor. For instance, a capacitive sensor element that is configured to detect an input, such as proximity or contact with a finger or other object, may have a capacitance CP between the sensor element and ground when no input is present. The capacitance CP is known as the parasitic capacitance of the sensor. For capacitive sensors having multiple sense elements, a mutual capacitance CM may also be present between two or more sense elements. An input detected by the sensor may cause a change in capacitance CF that is much smaller than CP or CM. Accordingly, where the sensor capacitance is represented as a digital code, the parasitic or mutual capacitances may be represented by a larger proportion of the discrete capacitance levels resolvable by the digital code, while the capacitance change CF is represented by fewer of these discrete levels. In such cases, the capacitance change CF due to an input may not be resolvable to a high degree of resolution.
Additionally, the design of some capacitive sensors also results in a high susceptibility to noise due to electromagnetic interference (EMI). For example, a capacitive touchpad or slider device may include an array of capacitive sensor elements, each of which may include a conductive trace having a substantial length. Such conductive traces may couple noise into a capacitance measurement circuit and reduce the ability of the measurement circuit to measure capacitance levels accurately and precisely.